As shown in FIG. 11, there has heretofore been available a dehumidifying apparatus having a compressor 1 for compressing a refrigerant, a condenser 2 for condensing the compressed refrigerant with outside air, an evaporator 3 for depressurizing the condensed refrigerant with an expansion valve 5 and evaporating the refrigerant to cool process air from an air-conditioned space 10 to a temperature equal to or lower than its dew point, and a reheater 4 for reheating the process air, which has been cooled to a temperature equal to or lower than its dew point, at the downstream side of the condenser 2 with the refrigerant upstream of the expansion valve 5. The refrigerant is condensed in the condenser and the reheater. With the illustrated dehumidifying apparatus, a heat pump HP is constituted by the compressor 1, the condenser 2, the reheater 4, the expansion valve 5, and the evaporator 3. The heat pump HP pumps heat from the process air which flows through the evaporator 3 into the outside air which flows through the condenser 2.
Here, operation of the heat pump HP shown in FIG. 11 will be described below with reference to a Mollier diagram shown in FIG. 12. The diagram shown in FIG. 12 is a Mollier diagram in the case where HFC134a is used as the refrigerant. A point a represents a state of the refrigerant evaporated by the evaporator 3, and the refrigerant is in the form of a saturated vapor. The refrigerant has a pressure of 0.34 Mpa, a temperature of 5° C., and an enthalpy of 400.9 kJ/kg. A point b represents a state of the vapor drawn and compressed by the compressor 1, i.e., a state at the outlet port of the compressor 1. In the point b, the refrigerant is in the form of a superheated vapor. The refrigerant vapor is cooled in the condenser 2 and reaches a state represented by a point c in the Mollier diagram. In the point c, the refrigerant is in the form of a saturated vapor and has a pressure of 0.94 MPa and a temperature of 38° C. Under this pressure, the refrigerant is cooled and condensed to reach a state represented by a point d. In the point d, the refrigerant is in the form of a saturated liquid and has the same pressure and temperature as those in the point c. The saturated liquid has an enthalpy of 250.5 kJ/kg. The refrigerant liquid is depressurized by an expansion valve 5 to a saturation pressure of 0.34 MPa at a temperature of 5° C. A mixture of the refrigerant liquid and the vapor at a temperature of 5° C. is delivered to the evaporator 3, in which the mixture removes heat from process air and is evaporated to reach a state of the saturated vapor, which is represented by the point a in the Mollier diagram. The saturated vapor is drawn into the compressor 1 again, and the above cycle is repeated.
Operation of the dehumidifying apparatus shown in FIG. 11 will be described below with reference to a psychrometric chart shown in FIG. 13. In FIG. 13, the alphabetical letters K, L, M correspond to the encircled letters in FIG. 11. Air (in a state K) from the air-conditioned space 10 is cooled to a temperature equal to or lower than its dew point to lower the dry bulb temperature thereof and lower the absolute humidity thereof, and reaches a state L. The state L is on a saturation curve in the psychrometric chart. The air in the state L is reheated by the reheater 4 to increase the dry bulb temperature thereof and keep the absolute humidity thereof constant, and reaches a state M. Then, the air is supplied to the air-conditioned space 10. The state M is lower in both of absolute humidity and dry bulb temperature than the state K.
With the conventional heat pump and dehumidifying apparatus described above, since it is necessary to considerably cool the air to its dew point, about half of the refrigerating effect of the evaporator in the heat pump is consumed to remove a sensible heat load from the air, so that the moisture removal (the dehumidifying performance) per electric power consumption is low. If a single-stage compressor is used as the compressor in the heat pump, then it produces a one-stage compression-type refrigerating cycle, resulting in a low coefficient of performance (COP) and a large amount of electric power consumed per amount of moisture removal.
It is therefore an object of the present invention to provide a heat pump with a high coefficient of performance (COP) and a dehumidifying apparatus which consumes a small amount of energy per amount of moisture removal.